Inductively coupled plasmas ("ICPs") generated with radio frequency ("RF") waves having a frequency generally between 1 MHz and 100 MHz are capable of providing charged particle (electron and ion) concentrations in excess of 10.sup.11 cm.sup.-3 and ion currents to wafer substrates in excess of 5 mA/cm.sup.2. The ICP source is thus competitive with electron cyclotron resonance ("ECR") plasma sources for semiconductor manufacturing processes requiring plasma generation. Semiconductor manufacturing processes that make use of plasmas include dry etching, plasma enhanced deposition, dry cleaning of wafers, and applications requiring the generation of ultraviolet (UV) light.
Inductively coupled RF plasma sources have advantages over both capacitively coupled RF plasma sources and ECR plasma sources. In contrast to capacitively coupled RF plasmas, inductively coupled RF plasmas have substantially lower intrinsic plasma potentials (&lt;50 V) and achieve a substantially higher ionization efficiency (&gt;5%). Also, the intrinsic plasma potential is relatively independent of the RF power. The low intrinsic plasma potential is useful in applications where high ion energies cannot be tolerated, such as in dry etching where high ion energies can damage the devices on the wafer.
In ECR plasma sources, the plasma ions are produced by electron bombardment in a discharge chamber, and directed towards the surface using magnetic and/or electric fields. As in the case of ECR systems, the ion energy of an inductively coupled RF plasma can be varied independently of the plasma density by biasing the integrated circuit wafer with a separate RF or DC power supply. For an ECR plasma source, the pressure at which the plasma may be effectively generated is also a concern. An ECR source is most effective at pressures below 1 mTorr, which is too low for most semiconductor process applications. The ICP source, however, has the advantage of operating over a pressure range that is more compatible with semiconductor process requirements (1 mTorr to 50 mTorr). Since the operating pressure is higher, the pumping requirements for a given gas flow rate are more modest for the ICP source. In addition, the ICP source can provide a larger diameter (15 cm to 30 cm), homogeneous plasma, in a compact design, and at substantially lower cost than an ECR source.
One type of plasma source employing RF induction coupling couples energy into the plasma through whistler or helicon waves. This type of generator is called a helicon plasma source. In the presence of a magnetic field ranging from 100 G to 1 kG directed along the axis of the source, a whistler wave can be excited by applying an RF voltage to a loop antenna located around the source cavity. Although these axial magnetic fields are generally weaker than the magnetic fields employed in ECR sources, the plasma is non-uniform across the diameter of the source. Thus, a wafer undergoing a plasma process must be located away or "downstream" of the source, in a region where the plasma is sufficiently uniform. This requires the input power of the source to be increased to maintain a sufficient plasma density (i.e., electron and ion concentration) at the downstream position. Also, large solenoidal coils are required to generate the axial magnetic field. These features increase source cost and complexity.
A second type of plasma source differs from the generic whistler wave or helicon source by omitting the axial magnetic field. The wafer undergoing a plasma process can therefore be placed within the plasma generation region. Even though the peak plasma densities (5.times.10.sup.11 cm.sup.-3) for such a source are about an order of magnitude lower than those for the whistler wave source, the proximity of the wafer to the plasma generation region in the source ensures that processing rates are comparable. Wafer etch rates of over 1 .mu.m/min are possible for many materials of interest. This source is simpler, more compact, and cheaper than the helicon plasma source.
One version of this type of induction plasma source employs a multi-turn pancake coil located along the top surface of a cylindrical vacuum chamber. A quartz vacuum window, typically 0.5 in. thick, isolates the coil from the chamber. When the coil is powered by an RF source, large currents circulate in the coils. These currents induce intense electric fields inside the chamber that sustain the plasma state. The time-varying magnetic and electric fields generated by the pancake coil are proportional to the coil current, and increase in proportion to the square of the number of coil turns and the coil diameter. The uniformity of the induced electric field from a pancake coil improves with increasing coil diameter and the number of coil turns. However, the inductance of the coil is also proportional to the square of the number of coil turns. This implies that the voltage drop across the coil increases with an increasing number of coil turns for a fixed coil current. As an example, the voltage drop across a 5 .mu.H coil for an RMS current of 20 A at 13.56 MHz is 8.5 kV. Such a high voltage is a hazard and results in capacitive energy coupling between the coil and the plasma. Capacitive coupling is undesirable because the intrinsic plasma potential increases dramatically if a significant amount of energy is transferred via capacitive coupling. These issues constrain the number of coil turns to about three in these RF plasma sources with multi-turn pancake coils located along the top surface of the source.